1. Field of the Invention:
The present invention relates to a vinyl chloride resin composition. More particularly, the present invention relates to a vinyl chloride resin composition which is molded into films or sheets and is suitable for agricultural uses as an excellent weather-proofing material.
2. Description of the Prior Art:
In recent years, with the recent advances in the modernization of agricultural techniques, plant cultivation in houses has been actively pursued, and, as covering materials for the houses used in plant cultivation, such materials as glass sheets, polyethylene films, polyvinyl chloride films, ethylene-vinyl acetate copolymer films, and the like have been used. At the present time, polyvinyl chloride films are preferably and predominantly used because of their excellent heat retaining properties, transparency, strength, workability, and the like, and because they are relatively inexpensive. However, when the polyvinyl chloride films are used out-of-doors for several months they generally become brown and deteriorate. Polyvinyl chloride films as used for molding purposes in agricultural techniques are normally stabilized by incorporating a light, heat and oxidation stabilizer such as an organotin compound, a metallic soap, a lead stabilizer, an organocadmium compound, an organobarium compound, an antioxidant, an ultraviolet absorbent, or the like in the films to prevent the deterioration of the molded films. However, these stabilizers have had to be used in relatively very large amounts to stabilize the polyvinyl chloride films. Of course, when the stabilizers are added to the films in large amounts, some of the stabilizers improve the weather-proofing capability of the molded articles, while their presence decreases such physical properties as the workability, transparency, or the like of the films. For example, if a large amount of a metallic soap stabilizer is blended with the polyvinyl chloride when it is molded, a phenomenon known as "plating out" occurs which spoils the appearance of molded article, decreases the processing efficiency and decreases the transparency of the film. Furthermore, for example, when a large amount of an ultraviolet absorbent is blended with the polyvinyl chloride, the molded article turns a yellow color so that when the film is used for agricultural purposes, the film absorbs the light of wavelengths which are necessary for plant growth. Thus, the growth of some types of plants is occasionally hindered, because the region of the spectrum which is absorbed, is generally within the range of 300 to 400 milimicrons. For these reasons improvements in the weather-proofing properties of polyvinyl chloride blended with these stabilizers have been difficult to obtain. One method of improving the weather-proofing properties of the films has been to incorporate a widely employed additive such as an epoxy compound or tricresylphosphate (hereinafter abbreviated as "TCP") to assist the stabilizers. These additives, particularly TCP, are very effective in improving the weather-proofing properties of the films. However, only a limited amount of the supplemental additive can be incorporated in the film. If these amounts are exceeded, detrimental things occur. For example in the case of epoxy resins, bleeding of the resin from the film occurs, i.e., a liquid material oozes out onto the surface of the molded film, and, as a consequence, the films adhere to each other which makes separation of the films more difficult and which increases the physical brittleness of the films. Also, excessive amounts of TCP in the molded polyvinyl chloride materials decreases their weather-proofing properties and often decreases their thermal resistance. Thus, when the conventional polyvinyl chloride films have been used for agricultural purposes out-of-doors, their useful lifetime is generally about one year, and at best no more than two years.
A need, therefore, continues to exist for a film which is suitable for agricultural purposes and which can be used out-of-doors for more than two years such as on plant cultivation houses.